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When the Curiosity rover arrived on the rocky planet, it may have had bacterial stowaways, says Stephanie Smith, a microbiologist at the University of Idaho. That may mean that if we do missions to detect life on Mars, we may find bacteria that evolved on earth.

Her group used swabs from Curiosity's surface before it was launched, finding over 60 species. The group cultivated 377 strains of bacteria, and tested them in extreme conditions: drying them out, dropping the temperature, bombarding them with UV radiation, and extremely alkaline environments. Almost 11 percent of bacteria survived at least one of these conditions.

The Outer Space Treaty of 1967 specifies that studies of space, including the Moon and our neighboring planets, should be conducted "so as to avoid their harmful contamination." What that means in practice is that there are limits on how many spore-forming bacteria that are allowed on the surface of the bacteria before the launch, Smith said. Spore forming bacteria, in extreme conditions, are capable of going dormant; many species of Bacillus share this trait. Their known toughness was why they're what's specified for. But on the swab from Curiosity, Smith and her team found other species that didn't form spores could survive some of these conditions as well.

"We were trying to ask, have we already contaminated Mars?" Smith said. "Or is there no way they could even survive, they'd die on the way because of the intense environment."

Spacecraft aren't sterilized before they leave the U.S. Rather, they undergo a process called "bioburden reduction," which uses a number of methods to kill off bacteria. Obviously, as the swabs analyzed by Smith's team show, not all of the bacteria are killed that way -- the main method for sterilization is dry heat, but some of the materials used to make this generation of spacecraft won't withstand it.

There are two worries for scientists if Earth bacteria do make the journey. First, if there's already life there, the Earth bacteria may serve as an invasive species, killing off or out-competing the natives. The second is that if a life-detection mission goes to Mars, it may find our hardiest hitchhikers, fooling scientists into thinking that life exists on Mars when it didn't before our meddling, Smith said.

New methods for spacecraft decontamination need to be developed before we send missions to detect life on Mars. Right now, our ships are too dirty to send to areas where life might be found on the Martian surface, Smith said.